The present invention relates to an internal combustion engine having a stiffened crankcase and a shaft bearing structure with a built-in lubricating oil supply system.
In some conventional internal combustion engines, a bearing cap which supports a lower portion of a journal for supporting a crankshaft against a crankcase is fastened to the crankcase (see Japanese Laid-Open Patent Publication No. 61-104114, for example).
The engine of such a construction should have a complex reinforcing structure to give a required degree of rigidity to the crankcase.
Some recent internal combustion engines have balancer shafts with balancer weights, the balancer shafts being rotatably supported in the cylinder block symmetrically with respect to the crankshaft. The balancer shafts are rotated at a speed which is twice that of rotation of the crankshaft to cancel out the inertial force due to the reciprocating mass of engine components such as pistons to reduce unwanted vibration. Since the balancer shafts are rotated at the higher speed than that of rotation of the crankshaft, the bearings of the cylinder block in which the balancer shafts are rotatably supported should be supplied with a large amount of lubricating oil.
One known bearing structure for such balancer shafts is disclosed in Japanese Patent Publication No. 54-17106. According to the disclosed bearing structure, balancer shafts are supported in a crankcase by seats, and lubricating oil is led to bearings for the balancer shafts through oil holes defined in the crankcase, oil holes defined in the seats, and oil holes defined in the balancer shafts.
The disclosed bearing structure has however been disadvantageous in that the oil holes must be machined in the crankcase, resulting in an increase in the cost of manufacture. The crankcase is generally formed by casting and hence tends to have cavities therein. Therefore, when the oil holes are subsequently machined in the crankcase, they may communicate with such cavities. With the oil holes joined to the cavities, oil leakage tends to occur during operation. The conventional bearing structure has thus been unreliable.